Effects of the hormones Prolactin and Somatotrophin on brain development will be studied in frogs and mice. Our preliminary results show that these hormones greatly increase brain weight, DNA, RNA and proteins if injected into frogs tadpoles. Proposed experiments will examine the effects of the hormones on the brain of fetal and newborn mice, by means of autoradiography and scintillation counting after H3- thymidine administration. The patterns of DNA synthesis, cell proliferation and cell death in various parts of the brain will be investigated. It is not known whether the hormones act on either neurons or glia, or on both types of brain cells. This will be ascertained by disaggregating and separating the brain cells, which will be identified by several criteria. Neither is it known whether the brain enhancement after hormone treatment is the result of an increased production or a diminished destruction of brain cells. The proposed experiments are designed to provide information about the mode of increase in brain cells as well as about the types of cells affected by the hormones.